Collision detection

ABSTRACT

An improvement to a code-division-multiple-access (CDMA) system employing spread-spectrum modulation, with the CDMA system having a base station (BS) and a plurality of remote stations. The base station has a BS-spread-spectrum transmitter and a BS-spread-spectrum receiver. A remote station has an RS-spread-spectrum transmitter and an RS-spread-spectrum receiver. The BS transmitter transmits a broadcast common-synchronization channel, which includes a frame-timing signal. The broadcast common-synchronization channel has a common chip-sequence signal, which is common to the plurality of remote stations. In response to the RS-spread-spectrum receiver receiving the broadcast common-synchronization channel, and determining frame timing from the frame-timing signal, an RS-spread-spectrum transmitter transmits an access-burst signal. The access-burst signal includes a collision-detection portion. In response to the BS-spread-spectrum receiver receiving the access-burst signal, the BS-spread-spectrum transmitter transmits an collision-detection signal with the collision detection portion. In response to the BS-spread-spectrum receiver not receiving the access-burst signal due to a collision with a collision access-burst signal, the BS-spread-spectrum transmitter transmits an collision-detection signal without the correct collision detection portion.

RELATED APPLICATIONS

This application is a Continuation of U.S. application Ser. No.10/412,576, filed on Apr. 14, 2003, which is a Continuation of U.S.application Ser. No. 09/273,450, filed on Mar. 22, 1999, the entirecontents of each of which are hereby incorporated by reference.

BACKGROUND OF THE INVENTION

This invention relates spread-spectrum communications, and moreparticularly to code-division-multiple-access (CDMA) cellular, collisiondetection for packet-switched systems.

DESCRIPTION OF THE RELEVANT ART

Presently proposed for a standard is a random-access burst structurewhich has a preamble followed by a data portion. The preamble has 16symbols, the preamble sequence, spread by an orthogonal Gold code. Amobile station acquires chip and frame synchronization, but noconsideration is given to closed-loop power control or collisiondetection.

SUMMARY OF THE INVENTION

A general object of the invention is to detect collisions for packetdata transfer on CDMA systems.

Another object of the invention is to maintain reliability for high datathroughput and low delay on CDMA systems.

An objective is to provide random channel access with reliable high datathroughput and low delay on CDMA systems

At a first RS-spread-spectrum receiver, the steps further includereceiving the broadcast common-synchronization channel. From thebroadcast common-synchronization channel, the steps include determiningframe timing at the first RS-spread-spectrum receiver from theframe-timing signal.

From a first RS-spread-spectrum transmitter, the steps includetransmitting an access-burst signal. The access-burst signal hasmultiple segments at different power levels, that is to say typically atsequentially increasing power levels a collision.

The BS-spread-spectrum receiver receives at least one segment of theaccess burst signal at a detectable power level. In response, theBS-spread-spectrum transmitter sends an acknowledgment signal back tothe first RS-spread-spectrum receiver. Receipt of the acknowledgmentsignal by the first RS-spread-spectrum receiver causes theRS-spread-spectrum transmitter to send data to the BS-spread-spectrumreceiver. The detection of the segment at an adequate power level,acknowledgment communication and subsequent data transmission providesthe remote station (RS) with random access to the channel (RACH).

The preferred embodiment also provides that when there is a collision ofa first access-burst signal with a collision access-burst signal, thenthe BS-spread-spectrum receiver does not correctly receive the collisiondetection portion of the first access-burst signal. Thus, theBS-spread-spectrum transmitter transmits to the first RS-spread-spectrumreceiver, an collision-detection without reflecting thecollision-detection portion. At the first RS-spread-spectrum receiver,in response to receiving the collision-detection signal without thecollision detection portion, the first RS-spread-spectrum transmittertransmits to the BS-spread-spectrum receiver, a second access-burstsignal.

Additional objects and advantages of the invention are set forth in partin the description which follows, and in part are obvious from thedescription, or may be learned by practice of the invention. The objectsand advantages of the invention also may be realized and attained bymeans of the instrumentalities and combinations particularly pointed outin the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are incorporated in and constitute apart of the specification, illustrate preferred embodiments of theinvention, and together with the description serve to explain theprinciples of the invention.

FIG. 1 is a common packet channel system block diagram with a commoncontrol downlink channel;

FIG. 2 is common packet channel system block diagram with a dedicateddownlink channel;

FIG. 3 is a block diagram of a base station receiver for common packetchannel;

FIG. 4 is a block diagram of a remote station receiver and transmitterfor common packet channel;

FIG. 5 is a timing diagram for access burst transmission;

FIG. 6 illustrates common packet channel access burst of FIG. 5 using acommon control downlink channel;

FIG. 7 illustrates common packet channel access of FIG. 5 using adedicated downlink channel

FIG. 8 shows the structure of the preamble;

FIG. 9 illustrates preamble and pilot formats;

FIG. 10 is a common packet channel timing diagram and frame format ofthe down link common control link; and

FIG. 11 illustrates frame format of common packet channel, packet data.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Reference now is made in detail to the present preferred embodiments ofthe invention, examples of which are illustrated in the accompanyingdrawings, wherein like reference numerals indicate like elementsthroughout the several views.

The common-packet channel is a new and novel uplink transport channelfor transmitting variable size packets from a remote station to a basestation within listening range, without the need to obtain a two waylink with any one or set of base stations. The channel resourceallocation is contention based; that is, a number of mobile stationscould at any time content for the same resources, as found in an ALOHAsystem.

In the exemplary arrangement shown in FIG. 1, common-packet channelprovides an improvement to a code-division-multiple-access (CDMA) systememploying spread-spectrum modulation. The CDMA system has a plurality ofbase stations (BS) 31, 32, 33 and a plurality of remote stations (RS).Each remote station 35 has an RS-spread-spectrum transmitter and anRS-spread-spectrum receiver. An uplink is from the remote station 35 toa base station 31. The uplink has the common-packet channel (CPCH). Adownlink is from a base station 31 to the remote station 35, and isdenoted a common-control channel (CCCH). The common-control channel hascommon signaling used by the plurality of remote stations.

An alternative to the common-control channel, but still using thecommon-packet channel, is the downlink dedicated physical channel(DPCH), shown in FIG. 2. The dedicated downlink channel, has signalingthat is used for controlling a single remote station.

As illustratively shown in FIG. 3, a BS spread-spectrum transmitter anda BS spread-spectrum receiver is shown. The BS spread-spectrumtransmitter and the BS spread-spectrum receiver are located at the basestation 31. The BS spread-spectrum receiver includes an antenna 309coupled to a circulator 310, a receiver radio frequency (RF) section311, a local oscillator 313, a quadrature demodulator 312, and ananalog-to-digital converter 314. The receiver RF section 311 is coupledbetween the circulator 310 and the quadrature demodulator 312. Thequadrature demodulator is coupled to the local oscillator 313 and to theanalog to digital converter 314. The output of the analog-to-digitalconverter 315 is coupled to a programmable-matched filter 315.

A preamble processor 316, pilot processor 317 and data-and-controlprocessor 318 are coupled to the programmable-matched filter 315. Acontroller 319 is coupled to the preamble processor 316, pilot processor317 and data-and-control processor 318. A de-interleaver 320 is coupledbetween the controller 319 and a forward-error-correction (FEC) decoder321.

The BS spread-spectrum transmitter includes a forward-error-correction(FEC) encoder 322 coupled to an interleaver 323. A packet formatter 324is coupled to the interleaver 323 and to the controller 319. A variablegain device 325 is coupled between the packet formatter 324 and aproduct device 326. A spreading-sequence generator 327 is coupled to theproduct device 326. A digital-to-analog converter 328 is coupled betweenthe product device 328 and quadrature modulator 329. The quadraturemodulator 329 is coupled to the local oscillator 313 and a transmitterRF section 330. The transmitter RF section 330 is coupled to thecirculator 310.

The controller 319 has control links coupled to the analog-to-digitalconverter 314, programmable-matched filter 315, preamble processor 316,the digital-to-analog converter 328, the spreading sequence generator327, the variable gain device 325, the packet formatter 324, thede-interleaver 320, the FEC decoder 321, the interleaver 323 and the FECencoder 322.

A received spread-spectrum signal from antenna 309 passes throughcirculator 310 and is amplified and filtered by receiver RF section 311.The local oscillator 313 generates a local signal which quadraturedemodulator 312 uses to demodulator in-phase and quadrature phasecomponents of the received spread-spectrum signal. The analog-to-digitalconverter 314 converts the in-phase component and the quadrature-phasecomponent to a digital signal. These functions are well known in theart, and variations to this block diagram can accomplish the samefunction.

The programmable-matched filter 315 despreads the receivedspread-spectrum signal. A correlator, as an alternative, may be used asan equivalent means for despeading the received spread-spectrum signal.

The preamble processor 316 detects the preamble portion of the receivedspread-spectrum signal. The pilot processor detects and synchronizes tothe pilot portion of the received spread-spectrum signal. The data andcontrol processor detects and processes the data portion of the receivedspread-spectrum signal. Detected data passes through the controller 319to the de-interleaver 320 and FEC decoder 321. Data and signaling areoutputted from the FEC decoder 321.

In the BS transmitter, data are FEC encoded by FEC encoder 322, andinterleaved by interleaver 323. The packet formatter formats data,signaling, acknowledgment signal, collision detection signal, pilotsignal and transmitting power control (TPC) signal into a packet. Thepacket is outputted from packet formatter, and the packet level isamplified or attenuated by variable gain device 325. The packet isspread-spectrum processed by product device 326, with a spreadingchip-sequence from spreading-sequence generator 327. The packet isconverted to an analog signal by digital-to-analog converter 328, andin-phase and quadrature-phase components are generated by quadraturemodulator 329 using a signal from local oscillator 313. The packet istranslated to a carrier frequency, filtered and amplified by transmitterRF section 330, and then passes through circulator 310 and is radiatedby antenna 309.

In the illustrative embodiment shown in FIG. 4, a RS spread-spectrumtransmitter and a RS spread-spectrum receiver are shown. The RSspread-spectrum transmitter and the RS spread-spectrum receiver arelocated at the mobile implementation of the remote station 35, shown asan MS (mobile station) in FIG. 1. The RS spread-spectrum receiverincludes an antenna 409 coupled to a circulator 410, a receiver radiofrequency (RF) section 411, a local oscillator 413, a quadraturedemodulator 412, and an analog-to-digital converter 414. The receiver RFsection 411 is coupled between the circulator 410 and the quadraturedemodulator 412. The quadrature demodulator is coupled to the localoscillator 413 and to the analog to digital converter 414. The output bfthe analog-to-digital converter 415 is coupled to a programmable-matchedfilter 415.

An acknowledgment detector 416, pilot processor 417 and data-and-controlprocessor 418 are coupled to the programmable-matched filter 415. Acontroller 419 is coupled to the acknowledgment detector 416, pilotprocessor 417 and data-and-control processor 418. A de-interleaver 420is coupled between the controller 419 and a forward-error-correction(FEC) decoder 421.

The MS spread-spectrum transmitter includes a forward-error-correction(FEC) encoder 422 coupled to an interleaver 423. A packet formatter 424is coupled through a multiplexer 451 to the interleaver 423 and to thecontroller 419. A preamble generator 452 and a pilot generator 453 forthe preamble are coupled to the multiplexer 451. A variable gain device425 is coupled between the packet formatter 424 and a product device426. A spreading-sequence generator 427 is coupled to the product device426. A digital-to-analog converter 428 is coupled between the productdevice 428 and quadrature modulator 429. The quadrature modulator 429 iscoupled to the local oscillator 413 and a transmitter RF section 430.The transmitter RF section 430 is coupled to the circulator 410.

The controller 419 has control links coupled to the analog-to-digitalconverter 414, programmable-matched filter 415, acknowledgment detector416, the digital-to-analog converter 428, the spreading sequencegenerator 427, the variable gain device 425, the packet formatter 424,the de-interleaver 420, the FEC decoder 421, the interleaver 423, theFEC encoder 422, the preamble generator 452 and the pilot generator 453.

A received spread-spectrum signal from antenna 409 passes throughcirculator 410 and is amplified and filtered by receiver RF section 411.The local oscillator 413 generates a local signal which quadraturedemodulator 412 uses to demodulate in-phase and quadrature phasecomponents of the received spread-spectrum signal. The analog-to-digitalconverter 414 converts the in-phase component and the quadrature-phasecomponent to a digital signal. These functions are well known in theart, and variations to this block diagram can accomplish the samefunction.

The programmable-matched filter 415 despreads the receivedspread-spectrum signal. A correlator, as an alternative, may be used asan equivalent means for despreading the received spread-spectrum signal.

The acknowledgment detector 416 detects the an acknowledgment in thereceived spread-spectrum signal. The pilot processor detects andsynchronizes to the pilot portion of the received spread-spectrumsignal. The data and control processor detects and processes the dataportion of the received spread-spectrum signal. Detected data passesthrough the controller 419 to the de-interleaver 420 and FEC decoder421. Data and signaling are outputted from the FEC decoder 421.

In the RS transmitter, data are FEC encoded by FEC encoder 422, andinterleaved by interleaver 423. The preamble generator 452 generates apreamble and the pilot generator 453 generates a pilot for the preamble.The multiplexer 451 multiplexes the data, preamble and pilot, and thepacket formatter 424 formats the preamble, pilot and data into acommon-packet channel packet. Further, the packet formatter formatsdata, signaling, acknowledgment signal, collision detection signal,pilot signal and TPC signal into a packet. The packet is outputted frompacket formatter, and the packet level is amplified or attenuated byvariable gain device 425. The packet is spread-spectrum processed byproduct device 426, with s spreading chip-sequence fromspreading-sequence generator 427. The packet is converted to an analogsignal by digital-to-analog converter 428, and in-phase andquadrature-phase components are generated by quadrature modulator 429using a signal from local oscillator 413.

Referring to FIG. 5, the base station transmits a common-synchronizationchannel, which has a frame time duration T_(F). Thecommon-synchronization channel has a common chip-sequence signal, whichis common to the plurality of remote stations communicating with theparticular base station. In a particular embodiment, the time T_(F) ofone frame is ten milliseconds. Within one frame, there are eight accessslots. Each access slot lasts 1.25 milliseconds. Timing for the accessslots is the frame timing, and the portion of thecollation-synchronization channel with the frame timing is denoted theframe-timing signal. The frame-timing signal is the timing a remotestation uses in selecting an access slot in which to transmit anaccess-burst signal.

A first remote station attempting to access the base station, has afirst RS-spread-spectrum receiver for receiving the commonsynchronization channel, broadcast from the base station. The firstRS-spread-spectrum receiver determines frame timing from theframe-timing signal.

A first RS-spread-spectrum transmitter, located at the first remotestation, transmits an access-burst signal. An access burst signal, asshown in FIG. 5, starts at the beginning of an access slot, as definedby the frame timing portion of the common-synchronization channel.

FIG. 6 illustratively shows the common-packet channel access burstformat, for each access-burst signal. Each access-burst signal has aplurality of segments. Each segment has a preamble followed by a pilotsignal. The plurality of segments has a plurality of power levels,respectively. More particularly, the power level of each segmentincreases with each subsequent segment. Thus, a first segment has afirst preamble and pilot, at a first power level P₀. A second segmenthas a second preamble and a second pilot, at a second power level P₁.The third segment has a third preamble and a third pilot at a thirdpower level P₂. The first preamble, the second preamble, the thirdpreamble, and subsequent preambles, may be identical or different. Thepower level of the pilot preferably is less than the power level of thepreamble. A preamble is for synchronization, and a corresponding pilot,which follows a preamble, is to keep the BS spread-spectrum receiverreceiving the spread-spectrum signal from the remote station, once apreamble is detected.

A subsequent increase or decrease of power levels is basically a closedloop power control system. Once a BS spread-spectrum receiver detects apreamble from the remote station, the BS spread-spectrum transmittersends an acknowledgment (ACK) signal.

Referring to FIG. 4, the preamble is generated by preamble generator 452and the pilot is generated by pilot generator 453. A preamble format isshown in FIG. 8. The preamble format with a pilot is shown in FIG. 9.The multiplexer 451, with timing from the controller 419, selects thepreamble then a corresponding pilot, for packet formatter 424. A seriesof preambles and pilots may be generated and made as part of the packetby packet formatter 424. The preambles and pilots can have their powerlevel adjusted either in the preamble generator 452 and pilot generator453, or by the variable gain device 425.

The BS spread-spectrum receiver receives the access-burst signal at adetected-power level. More particularly, the access-burst signal has theplurality of preambles at a plurality of power levels, respectively.When a preamble with sufficient power level is detected at the BSspread-spectrum receiver, then an acknowledgment (ACK) signal istransmitted from the BS spread-spectrum transmitter. The ACK signal isshown in FIG. 6, in response to the fourth preamble having sufficientpower for detection by the BS spread-spectrum receiver.

FIG. 3 shows the preamble processor 316 for detecting the preamble andthe pilot processor 317 for continuing to receive the packet afterdetecting the preamble. Upon detecting the preamble, the processor 319initiates an ACK signal which passes to packet formatter 324 and isradiated by the BS spread-spectrum transmitter.

The first RS-spread-spectrum receiver receives the acknowledgmentsignal. Upon receiving the ACK signal, the first RS-spread-spectrumtransmitter transmits to the BS-spread-spectrum receiver, aspread-spectrum signal having data. The data is shown in FIG. 6, intime, after the ACK signal. The data may include a collision detection(CD) portion of the signal, referred to herein as a collision detectionsignal, and message.

In response to each packet transmitted from the RS spread-spectrumtransmitter, the BS receiver detects the collision detection portion ofthe data, and retransmits the data field of the collision detectionportion of the data to the remote station. FIG. 10 shows the timingdiagram for re-transmitting the collision detection field. There areseveral slots for collision detection retransmission, which can be usedfor re-transmitting the collision detection field for several remotestations. If the collision detection field were correctly re-transmittedto the remote station, then the remote station knows its packet issuccessfully received by the base station. If the collision detectionfield were not correctly re-transmitted by the base station, then theremote station assumes there is a collision with a packet transmitted byanother remote station, and stops further transmission of the data. FIG.11 shows a frame format of a common-packet channel data payload.

In operation, an overview of the way this transport mechanism is used isas follows. A remote station (RS) upon power up searches fortransmission from nearby base stations. Upon successful synchronizationwith one or more base stations, the Remote station receives thenecessary system parameters from a continuously transmitted by all basestations broadcast control channel (BCCH). Using the informationtransmitted from the BCCH, the remote station can determine variousparameters required when first transmitting to a base station.Parameters of interest are the loading of all the base station in thevicinity of the remote station, their antenna characteristics, spreadingcodes used to spread the downlink transmitted information, timinginformation and other control infatuation. With this information, theremote station can transmit specific waveforms in order to capture theattention of a nearby base station. In the common packet channel theremote station, having all the necessary information from the nearbybase station, it starts transmitting a particular preamble from a set ofpredefined preambles, at a well selected time intervals. The particularstructure of the preamble waveforms is selected on the basis thatdetection of the preamble waveform at the base station is to be as easyas possible with minimal loss in detectability.

The physical common packet channel (CPCH) is used to carry the CPCH. Itis based on the well known Slotted ALOHA approach. There is a number ofwell defined time offsets relative to the frame boundary of a downlinkreceived BCCH channel. These time offsets define access slots. Thenumber of access slots is chosen according to the particular applicationat hand. As an example, shown in FIG. 5, eight access slots are spaced1.25 msec apart in a frame of 10-msec duration.

According to FIG. 5, a remote station picks an access slot in a randomfashion and tries to obtain a connection with a base station bytransmitting a preamble waveform. The base station is able to recognizethis preamble, and is expecting its reception at the beginning of eachaccess slot. The length of the access burst is variable and the lengthof the access burst is allowed to vary from a few access slots to manyframe durations. The amount of data transmitted by the remote stationcould depend on various factors. Some of those are: class capability ofthe remote station, prioritization, the control information transmitteddown by the base station, and various bandwidth management protocolsresiding and executed at the base station. A field at the beginning ofthe data portion signifies the length of the data.

The structure of the access burst is shown in FIG. 6. The access burststarts with a set of preambles of duration T_(p) whose power isincreased in time from preamble to preamble in a step-wise manner. Thetransmitted power during each preamble is constant. For the durationT_(D) between preambles the access burst consists of a pilot signaltransmitted at a fixed power level ratio relative to the previouslytransmitted preamble. There is a one to one correspondence between thecode structure of the preamble and the pilot signal. The pilot signalcould be eliminated by setting it to a zero power level.

The transmission of the preambles ceases if the preamble has been pickedup, detected, by the base station, and the base station has responded tothe remote station with a layer one acknowledgment L1 ACK which theremote station has also successfully received. Alternatively,transmission of the preamble ceases if the remote station hastransmitted the maximum allowed number of preambles M_(p) withoutacknowledgement. Upon receiving this L1 ACK the remote station startstransmission of its data. Once the remote station has transmitted morethan M_(p) preambles, it undergoes a forced random back off procedure.This procedure forces the remote station to delay its access bursttransmission for a later time. The random back off procedure could beparameterized based on the priority statues of the Remote station. Theamount by which the power is increased from preamble to preamble isD_(p) which is either fixed for all cells at all times or it isrepeatedly broadcast via the BCCH. Remote stations with differentpriority statuses could use a power increase which depends on a prioritystatus assigned to the remote station. The priority status could beeither predetermined or assigned to the remote station after negotiationwith the base station.

The Preamble Signal Structure

There is a large set of possible preamble waveforms. Every base stationis assigned a subset of preambles from the set of all preamble waveformsin the system. The set of preambles a base station is using is broadcastthrough it's BCCH channel. There are many ways of generating preamblewaveforms. One existing way is to use a single orthogonal Gold code perpreamble from the set of all possible orthogonal Gold codes of length L.A preamble could then be constructed by repeating the Gold code a numberof times N to transmit a length N complex sequence. For example if Adenotes the orthogonal Gold code and G_(i)={g_(i,0) g_(i,1) g_(i,2) . .. g_(i, N-1)}, a length N complex sequence, then a preamble could beformed as shown in FIG. 8, where, g_(i, j), j=0, . . . , N-1, multipliesevery element in A. Normally the sets of G_(i)'s are chosen to beorthogonal to each other. This will allow for a maximum of N possiblewaveforms. The total number of possible preambles is then L*N.

The preferred approach is to use different codes rather than a singlerepeating code in generating each preamble. In that case, if L possiblecodes, not necessarily Gold Codes, were possible, designated by A₀, A₁,. . . A_(L-1), then possible preambles will be as shown in FIG. 8. Theorder of the A_(i)'s can be chosen so that identical codes are not usedin the same locations for two different preambles. A similar approachcould be used to form the pilot signals.

The Downlink Common Control Channel

In FIG. 10, the downlink common control channel structure for even andodd slots is shown. The even slots contain reference data and controldata. The pilot symbols are used to derive a reference for demodulatingthe remaining control symbols. The control symbols are made of transportframe indicator (TFI) symbols, power control (PC) symbols, collisiondetection (CD) symbol and signaling symbols (SIG). The odd slots containall the information that the even slots contain plus an acknowledgment(ACK) signal. Odd slots do not include collision detection fields.

The uplink CPCH is shown over the last transmitted preamble. After thelast transmitted preamble, the base station has successfully detectedthe transmission of the last transmitted preamble and transmits back theacknowledgment signal. During the same time, the remote station is tunedto receive the ACK signal. The ACK signal transmitted corresponds to thespecific preamble structure transmitted on the uplink. Once the remotestation detects the ACK signal corresponding to transmitted preamble bythe remote station, the remote station begins transmission of its data.

Corresponding with the preamble structure in the uplink there is acorresponding in time power control information symbol and acorresponding in time collision detection field. Upon start of datatransmission the remote station uses the downlink transmitted powercontrol information to adjust its transmitted power. The power controlsymbols are decoded to derive a binary decision data, which is then usedto increase or decrease the transmitted power accordingly. FIG. 11 showsthe structure of the uplink frame and the slot format for the dataportion of the uplink transmission. Data and control information istransmitted in an in-phase and quadrature-phase multiplexed foijuat.That is, the data portion could be transmitted on the in-phasecoordinate and the control portion on the quadrature-phase coordinate.The modulation for the data and control is BPSK. The control channel maycontain the information for the receiver to enable the demodulation ofthe data. The control channel provides for upper layer systemfunctionality. The data portion consists of one or more frames. Eachframe consists of a number of slots. As an example the frame durationcould be 10 milliseconds long and the slot duration 0.625 millisecondslong. In that case, there are 16 slots per frame. The beginning of thedata payload contains a collision detection field used to relayinformation about the possibility of collision with other simultaneouslytransmitting remote stations. The collision detection field is read bythe base station. The base station expects the presence of the collisiondetection field since it had provided an ACK signal at the last timeslot.

The collision detection field includes a temporary identification (ID)number chosen at random by the mobile for the transmission of thecurrent packet. The base station reads the collision detection field andreflects, or transmits back, the collision detection field on thedownlink. If the collision detection field detected by the remotestation matched the one just being transmitted by the same remotestation, then the collision detection field is an identification thatthe transmission is being received correctly. The remote station thencontinues transmitting the remaining of the packet. In case thecollision detection field has not been received correctly by the remotestation, then the remote station considers the packet reception by thebase station as erroneous and discontinues transmission of the remainingpacket.

The function of the remaining fields are as follows. The Pilot fieldenables the demodulation of both the data and control bits. Thetransmitted power control (TPC) bits are used to control the power of acorresponding downlink channel, in case a down link channel directed tothe same user is operational. If the downlink channel were notoperational, then the TPC control bits can be used to relay additionalpilot bits instead.

The Rate Information (RI) field is used to provide the transmitter withthe ability to change its data rate without the necessity to explicitlynegotiate the instantaneous data rate with the base station. The servicefield provides information of the particular service the data bits areto be used for. The length field specifies the time duration of thepacket. The signal field can be used to provide additional controlinformation as required.

Additional functionalities of the common packet channel are: (1)bandwidth management and (2) L2 acknowledgment mechanism.

The bandwidth management functionality is implemented via signalinginformation on the down link common control channel. There are threeways for incorporating this functionality. The first relies on changingthe priority status of all uplink users, which currently aretransmitting information using the CPCH. By this method all the usersare remapping their priority status via a control signal sent at thedownlink. When the priority of the CPCH users is lowered their abilityto capture an uplink channel is lowered. Thus the amount of data sent onthe uplink by the CPCH users is thus reduced. The other mechanism is forthe base station to relay the maximum possible data rate the CPCH usersare allowed to transmit. This prevents the CPCH users from transmittingat a rate which could possibly exceed the uplink system capacity andtherefore take the cell down, i.e., disrupt the communication for allusers currently connected to the base station. For the third method, thebase station could provide a negative acknowledgment through the ACKsignal. In this case, any remote station which is tuned to receive theACK signal is prohibited from further transmission of an access-burstsignal.

The L2 acknowledgment (L2 ACK) mechanism, which is different than the L1ACK, is used by the base station to notify the remote station for thecorrectness of an uplink packet reception. The base station could eitherrelay to the remote station which portions of the packet have beingreceived correctly or which have being received incorrectly. There aremany existing ways of implementing a particular protocol to relay thistype of information. For example, the packet could be identified asconsisting of a number of frames, with each frame consisting of a numberof sub-frames. The frames are identified by a predetermined number. Thesub-frames in each frame are also identified by a specific number. Oneway for the base to relay the information about the correctness of thepacket is to identify all the frames and sub-frames that have beenreceived correctly. Another way is to identify the frames and sub-framesthat have been received in error. The way the base station couldidentify the correctness of a frame or sub-frame is by checking itscyclic residue code (CRC) field. Other more robust mechanisms foracknowledgment may be used.

CD Operation

There are many remote stations that might try to access the base stationat the same time. There is a number of different preamble signals whicha remote station can use for reaching the base station. Each remotestation chooses at random one of the preamble signals to use foraccessing the base station. The base station transmits a broadcastcommon synchronization channel. This broadcast common synchronizationchannel includes a frame timing signal. The remote stations extract theframe timing transmitted by the base station by receiving the broadcastcommon synchronization channel. The frame timing is used by the remotestations to derive a timing schedule by dividing the frame duration in anumber of access slots. The remote stations are allowed to transmittheir preambles only at the beginning of each access slot. The actualtransmit times for different remote stations could be slightly differentdue to their different propagation delays. This defines an accessprotocol commonly known as the slotted ALOHA access protocol. Eachremote station repeatedly transmits its preamble signal until the basestation detects the preamble, acknowledges that the preamble isreceived, and the acknowledgment is correctly received by the remotestation. There could be more than one remote station transmitting thesame preamble signal in the same access slot. The base station cannotrecognize if two or more remote stations were transmitting the samepreamble in the same access slot. When the base station detects thetransmission of a preamble signal, it transmits back an acknowledgmentmessage. There is one acknowledgment message corresponding to eachpossible preamble signal. Therefore, the are as many acknowledgmentmessages as there are preamble signals. Every transmitting remotestation which receives an acknowledgment message corresponding to itstransmitting preamble signal, will start transmitting its message. Foreach preamble signal, there is a corresponding spreading code used bythe base station to transmit the message. The message transmissionalways starts at the beginning of an access slot. Since there could be anumber of remote stations using the same preamble signal in the sameaccess slot, they start transmitting their message at the same timeusing the same spreading code. In that case, the transmissions of theremote stations likely interferes with each other and thus is notreceived correctly.

Each remote station includes a collision detection (CD) field in thebeginning of the transmitted message. The CD field is chosen at randomby each remote station and independently from each other Remote Station.There is a predefined limited number of CD fields. Two remote stationstransmitting their message at the same time most likely chose adifferent CD field. When the base station receives the CD field, thebase station reflects back, transmits back, the CD field to the remotestation. The remote station reads the reflected CD field by the basestation. If the reflected CD field matched the the CD field the remotestation transmitted, the the remote station assumes that the remotestation is being received correctly by the base station and continuetransmitting the rest of the message, or data. If the reflected CD fieldfrom the base station did not match the one transmitted by the remotestation, then the remote station assumes that there has been a collisionand stops transmitting the remaining message or data.

It will be apparent to those skilled in the art that variousmodifications can be made to the collision detection system of theinstant invention without departing from the scope or spirit of theinvention, and it is intended that the present invention covermodifications and variations of the collision detection system providedthey come within the scope of the appended claims and their equivalents.

1. An improvement to a code-division-multiple-access (CDMA) systememploying spread-spectrum modulation, with the CDMA system having a basestation (BS) with a BS-spread-spectrum transmitter and aBS-spread-spectrum receiver, and a plurality of remote stations, witheach remote station (RS) having an RS-spread-spectrum transmitter and anRS-spread-spectrum receiver, the method comprising the steps of:transmitting from said BS-spread-spectrum transmitter located at saidbase station, a broadcast common-synchronization channel having a commonchip-sequence signal common to the plurality of remote stations, thebroadcast common-synchronization channel having a frame-timing signal;receiving at a first RS-spread-spectrum receiver the broadcastcommon-synchronization channel, and determining frame timing at saidfirst RS-spread-spectrum receiver from the frame-timing signal;transmitting from a first RS-spread-spectrum transmitter an access-burstsignal, the access-burst signal having a collision-detection portion;receiving at said BS spread-spectrum receiver the access-burst signal,including the collision-detection portion; transmitting from saidBS-spread-spectrum transmitter to said first RS-spread-spectrumreceiver, responsive to receiving the access-burst signal with thecollision-detection portion, a collision-detection signal, thecollision-detection signal including the collision-detection portionfrom the access-burst signal; receiving at said first RS-spread-spectrumreceiver the collision-detection signal with the collision detectionportion; and transmitting from said first RS-spread-spectrumtransmitter, responsive to receiving the collision-detection signal, tosaid BS-spread-spectrum receiver, a spread-spectrum signal having data.2-4. (canceled)